This invention relates to telephone line circuits and more particularly to an arrangement for reducing crosstalk sensitivities in an inductorless circuit and the improvement of loop range in such circuits.
A telephone line circuit is interposed between the PBX line and the associated telephone station. These circuits serve, among other things, to provide both AC and DC power to activate the telephone transmitter and receiver.
Most telecommunications systems have a -48 volt DC source of power feeding power to the telephone stations. This is required by system interfaces to the external environment such as tie trunks, ground start trunks, and off premises stations. However, in situations such as PBX'S where on-premises stations are used on short loops from the PBX, -24 volts could replace the traditional -48 volt supply thereby saving power and component ratings. Complexity can be further reduced by using a -24 volt resistive line feed circuit. This is possible because a 24 volt resistive line feed circuit with a battery feed resistance of 600 ohms will supply an adequate amount of DC loop current to an on-premises station set and, at the same time, provide the required 600 ohm AC impedance to the on-premises station. The addition of a -24 volt supply increases power supply costs and adds cost to the power distribution system. For these reasons, the ability of the the our line feed circuit to efficiently supply 24 volt line feeds from a 48 volt supply, without the need for switching converters, is attractive, and, for controlled environment line loops, is commercially attractive.
Traditionally, line circuits would contain inductors or complex electronic circuitry which serve to isolate the DC power supply and to allow for the circuit's AC impedance to differ from the DC resistance. One major aim of circuit designers has been the elimination of the costly, and bulky, inductors or electronic components from the line circuits, thereby making the circuits fully resistive, i.e., where the DC resistance is the same as the AC impedance. In co-pending patent application D. C. Smith, Ser. No. 633,478, filed July 23, 1984, now U.S. Pat. No. 4,554,412 and having the same assignee as this application, there is disclosed and claimed a pair of resistive line circuits operating between -48 volts and ground. While these circuits operate properly, they are sensitive to the placement of, and wiring to, the -24 volt point. This sensitivity, which can result in excessive induced voltage signals (crosstalk) between lines, and one proposed arrangement for overcoming the problem, is discussed in the aforesaid patent application. However, since the proposed solution is highly dependent upon impedance to ground at the -24 volt point, a power bus can not be utilized, thus requiring individual wires to be run from each circuit to the common -24 volt power point. Accordingly, a need exists in the art for a resistive line circuit with improved crosstalk margins while at the same time allowing the use of a power bus common to several such circuits.
Another critical factor with resistive line circuits is the length of the line, known as the loop length, between the circuit and the associated telephone. A critical factor in determining loop length is the magnitude of available current from the line circuit. In the aforementioned disclosure, a modulated DC current source was used which subtracted from the available current and which resulted in a loop length of approximately two thousand feet of 24-gauge cable to a standard telephone set. Such a length is certainly acceptable in a majority of instances but situations do exist where it is desirable to increase the loop length beyond that range. Accordingly, a need exists in the art for a resistive line circuit where current available to the line is maximized.